Filter



May 11, 1931. R. s. YODER Er AL ,08

FILTER Filed Jan. 22, 1935 JLI mond iZ/ader Patented May 11, 1937 UNITED STATES FILTER Raymond S. Yoder and Raymond E. Wood, Chicago, Ill., assignors to Galvin Manufacturing Corporation, Chicago, 111.,

Illinois a corporation of Application January 22, 1935, Serial No. 2,904

4 Claims.

Our invention relates in general to filter circuits, and more in particular to a filter connected into a radio receiver circuit between the signal antenna and first demodulator of the receiver 5 designed to eliminate local electrical interference such as motor ignition noises in an automobile or aeroplane radio receiver.

One of the objects of our invention is the provision of an improved filter system for automobile radio receivers, aeroplane radio receivers and the like.

A further object of our invention is the provision of a simple, sturdy and inexpensive filter for the purposes enumerated above.

One of the features of our invention is the provision of a combination of inductances and capacitances assembled substantially in a unit and connected into the radio receiver circuit between the signal antenna and first demodulator s of the receiver.

A further feature is the use of an inductance coil in the filter comprising a double helix-shaped single wire coil adapted to be collapsed to lie in a shallow insulating cup to provide a very effective and sturdy member, yet inexpensive from the standpoint of material and labor.

Other objects and advantages of our invention .will be apparent from the following description taken with the drawing, in which:

30 Fig. 1 is a diagrammatic circuit arrangement showing the filter connected between the signal antenna of the radio receiving set and the input impedance.

Fig. 2 is a modified view of the filter circuit of Fig. 1 connected to a low impedance input.

"" Fig. 3 is a diagrammatic circuit arrangement of the filter of Fig. 2 terminated in a capacitance rather than in an inductance as shown in Fig. 2. Fig. 4 is top plan view of one of the filter sections.

Fig. 5 is a top plan view of one of the inductances employed in the filter.

Fig. 6 is a side elevation of one of the inductances of Fig. 5 in an expanded form; and

Fig. '7 is a diagrammatic circuit arrangement showing the filter connected between a radio fre quency stage and the first detector.

Referring to the circuit arrangement of Fig. 1, we provide a multi-section low-pass filter comprising a series of inductances l0, ll, 12 and i3 connected together in series, and in series connection with the signal or broadcast receiving antenna I4, as well as with the high impedance primary 16 of a radio frequency transformer whose secondary is the grid resonant circuit inductance of the input tube of the receiving set.

A shielding l5 extends over the conductor from the antenna M to the receiver. Shunt condensers l'I-l8 are connected to inductance H), with one (go plate of each condenser connected to the grounded shielding container 19. Each of the inductance coil sections is separated and shielded by a partition 2| comprising a part of the housing casing for the entire filter unit. A single condenser 22 is connected with inductance I l in the manner described, and a condenser 23 is likewise used in the third section of the filter unit. The filter unit is preferably mounted in the receiver housing, although it is understood that it may be assembled and shielded as will be hereinafter explained, and mounted externally of the receiver housing.

As will be noted from a consideration of Fig. I, the filter is terminated in the high impedance primary l6. Capacity coupling utilizing the capacitance 24 is employed between the filter output and the grid of the input tube of the receiver to compensate for the loss in gain at the high frequency end of the broadcast band when this type of primary is used. This type of termination for the filter is advantageous in that its high impedance matches the filter output impedance when low values of shunt capacity are used.

Fig. 2 illustrates a further embodiment and the preferred form of our invention, comprising a plurality of filter sections 25. This particular embodiment includes inductances 26, 21 and 28 connected in series with one another and in series connection between the signal antenna l4 and the input coil primary 29 of low impedance. Similarly to the embodiment of Fig. 1, the secondary of the radio frequency transformer including the primary 29 is the grid resonant circuit inductance of the input tube of the radio receiver circuit. The coil 29 is of the low impedance type and matches the output impedance of the filter at approximately the middle of the broadcast band in order to reduce the mis-match in impedance which will occur at either end of the band. This filter includes shunt condensers 3| and 32 connected to inductance 2'! and having one plate of each connected to the grounded shielding container IS. The various sections of the filter are likewise separated and shielded by a partition 2| comprising a part of the casing I9.

In the embodiment of Fig. 3, a filter circuit similar to that of Fig. 2 is employed. This circuit, however, has a different method of coupling the filter to the grid resonant circuit of the receiver. Capacitance 33 becomes the terminating impedance of the filter with condenser or capacitance 34 simply an isolating capacitance of negligible impedance. This is ordinarily known as the capacitive type of coupling. We have found that this particular method of coupling is advantageous in that larger constants can be used in the filter sections without disturbing the continuous alignment of the first reso nant circuit. Furthermore, instead of the terminating impedance 33 presenting a rising impedance with increasing frequency, the impedance is actually decreased, thereby presenting a low impedance to ground for the high frequency components of the ignition interference. It sometimes happens that the filter constants of this circuit may disturb the automatic volume control network, since the terminating impedance is the automatic volume control by-pass and the automatic volume control network is shunted by the filter constants. This difiiculty, however, is avoided by the use of the primary 29 as in Fig. 2.

Inasmuch as the interference frequencies from an ignition system are high frequencies, mostly,

' if not entirely, above the received frequency spectrum, and our device is a low pass filter, the interference is cut off above the received band.

Fig. 4 shows the actual physical structure of two sections of the filter. These are illustrative of any additional sections disclosed in Figs. 1 to 3. The section is enclosed by the shielding container l9 and is divided by a metal partition 2! separating inductances which for convenience We will refer to as 27 and 28, inasmuch as they are illustrative of the sections designated by those reference characters in Fig. The inductance coil in the member 21 is housed in an insulating cup-shaped casing 37 with a closing top 38 of in sulating material. The input to the filter is designated by I, and the output by O. The members 2'! and 28 are each mounted from the metal partition 2i on wooden dowels 39 and 4! respectively. The capacitances corresponding to 3i and 32 of Fig. 2 consist of fiat metal plates 31 fastened to the partition 2| but insulated therefrom by an interposed dielectric 43. A copper insulated conductor M extends from the input I to one electrode of the capacitance 3|. A sec- 0nd conductor 46 extends from the insulated electrode oi" capacitance 3i to a terminal on the inductance member 21. The other terminal of inductance member 2? carries an insulated cop per conductor 4? extending to the insulated electrode of condenser 32. The other electrode oi condenser 32 is connected by an insulated conductor 48 to the next succeeding filter section employing the member 28.

A modification of the above structure is indicated by dotted lines in Fig. 4, illustrating two mica condensers 48 and 5! connected across two terminals of inductance 2! and connected to an intervening common ground. In using these condensers, a connection is then made from the input terminal I directly to the lower terminal of the inductance 2?, and from there a connection extends to the condenser 49. A connection is made directly from the condenser 5| to the upper terminal of the member 21. These condensers when used are of course substituted for condensers 3| and 32 as illustrated in Fig. i.

In all of the embodiments of the filter, a common connection is provided for the shunt capacitances and input impedance such as to prevent a difierence of potential arising between these points. The connection may be accomplished by a common bus, or connection to the case as close together as possible. Preventing a difierence of potential between points of connection maintains the filter at high operating efiiciency.

Fig. 7 illustrates a further embodiment of the invention, showing the filter unit of Fig, 4 connected into the receiver circuit between a radio frequency amplifier A and the first detector stage B of the circuit. Although the previous embodiments have illustrated the filter unit as connected into the receiver circuit directly between the signal antenna and the input tube, it is understood that this unit may be introduced into the circuit any place before detection takes place.

An inductance coil such as If), for instance, which is of the same construction as the coils in each of the filter sections of all of the embodiments is illustrated in Figs. 5 and 6. The coil comprises a single wire which is extruded in a double-spiral form. The wire is insulated over its length, and in the commercial embodiment very satisfactory results have been obtained with enamel insulation. The coil is illustrated in Fig. 6 in its extended form, but upon final assembly is collapsed to fit into the shallow cup as shown in Fig. 4. Although ordinarily there is no danger of damaging the coil in the assembly thereof, to insure against scratching of the enamel a thin insulation 52 may be inserted as indicated at 52 before coilapsing the two portions. The coils in all provide a very efiicient, low cost inductance which lend themselves readily to production assembly.

Although we have described our invention in its preferred embodiments, it is understood that we are not limited thereby, but limit our invention only by the scope of the appended claims.

We claim:

1. In a filter unit for radio systems, an enclosing shielding container, an inductance coil insulatingly mounted in said container comprising a double spiral shaped coil, compressed to a substantially flat double layer coil for mounting in a shallow insulating casing, shunt condensers connected to said coil, and means for connecting said unit into the radio receiver circuit.

2. In a filter unit for radio systems, an enclosing shielding container, an inductance coil insulatingly mounted in said container and comprising a single piece of wire wound in the form of two fiat spirals compressed together for mounting in a shallow insulating casing, shunt condensers connected to said coil, and means for connecting said unit into the radio receiver circuit.

3. In a filter unit for radio systems, an enclosing shielding container, an inductance coil insulatingly mounted in said container and comprising a single piece of wire wound in the same direction throughout and forming an inwardly wound fiat spiral and an outwardly wound fiat spiral adjacent to one another, a shallow insulating casing housing said coil, a shunt condenser connected to said coil, and means for connecting said unit into the radio receiver circuit.

4. In a filter for a radio receiver system, an inductance coil connected into the receiver circuit for said system, shielding means for said coil, said inductance coil comprising a double spiral shaped coil compressed to a substantially flat double layer coil, and a shallow insulating mounting for insulatingly supporting said coil.

RAYMOND S. YODER. RAYMOND E. WOOD. 

